The object of this invention is a pulling device for providing a smooth linear pulling movement, comprising a shaft rotatable around the longitudinal axis thereof, a pulling nut moving linearly on the shaft and propelled by the rotational movement of the shaft and to which the member to be pulled is arranged to be attached, and a motor providing the rotational movement of the shaft.
The pulling device according to the invention is intended for application objects requiring extremely smooth linear movement. An example of such an application is the withdrawal of the sliding plate of a RIM cassette used in X-ray photography for reading the information recorded on the plate.
The X-ray radiation conducted through the object of the X-ray photography and modified by the absorption in the object is focused on the layer of tunable fluorescent material on the sliding plate of the RIM cassette with the information included on the radiation being recorded on the layer. After this exposure phase, the plate in the cassette is read by a separate reading device which converts, with the aid of a laser beam, the information into an image which is then displayed on a computer screen or written out on film.
The reading of the sliding plate of the RIM cassette is in practice effected by pulling the plate out of the cassette to the reading position where the information on the plate is read while the plate is in the smoothest possible linear movement. After the reading, the plate is transferred to an emptying position where it is emptied of information with the aid of a powerful light, and finally the plate is returned back to the cassette to wait for the following time of use. Previously the sliding plate has been transferred from the cassette to a separate belt conveyor for the reading and emptying of the information, but patent application FI-903796 discloses a newer solution which uses a pulling device which propels the sliding plate and pulls the plate out of the cassette and returns it back in while the reading and emptying of the information takes place during the reciprocating movement of the plate. The pulling device comprises a shaft driven by a stepper motor and rotatable around its longitudinal axis, and a pulling nut (slide) moving linearly on the shaft and engaging the sliding plate. The pulling nut is actuated by the rotational movement of the shaft through the intermediary of pulling bearings set at a pitch angle against the shaft.
Patent application FI-903796 does not closely describe the coupling of the stepper motor with the rotatable shaft. However, the solution heretofore included the coupling of the stepper motor to a flywheel via a massive rubber vibration absorber. The end of the shaft was attached to the flywheel by a screw joint. However, the difficulty with this kind of solution is that the flywheel and the shaft must be accurately concentric in order to avoid mutual lateral deviations in the rotational movements of these members. This has been a very difficult task to achieve in practice. In addition, the potential inclining of the shaft of the stepper motor has had, in the known solution, a tendency to correspondingly incline the shaft because of the limited flexibility of the massive rubber.